Typically, heat removal from industrial chemical, thermal or nuclear processes is achieved through evaporative cooling in a cooling circuit such as a cooling tower. As best illustrated in FIGS. 1 and 2, a typical type of cooling circuit is one including a cooling tower 1. As best depicted in FIG. 1, cooling water trickles down through a fill media 6 in the cooling tower 1, falls as rain 8, and accumulates in a basin 3. Makeup water 5 is often added to the basin 3 or other point in the cooling water circuit. Cooling water is pumped from the basin 3 via pump 7 to heat exchange system 9 where heat is exchanged with an industrial physical, chemical, or nuclear process.
As best illustrated in FIG. 1, one type of cooling tower includes water distribution equipment discharging cooling water over a fill media 6 (also called a packing). The cooling water trickles over the high surface area fill media 6 thereby allowing a greater degree of evaporation of the cooling water from thin films of it on the fill media 6 surfaces. A flow of air accelerating the evaporation is provided in one of two ways: natural draft or mechanical draft. In a natural draft system, due to the lower density of warmer air underneath the cooling tower (in comparison to cooler air outside the cooling tower at the same pressure), a flow of air 4 rises up through the fill media 6. In a mechanical draft system, a flow of air 4 is directed either up through or across the fill media 6 with the aid of blowers (not illustrated). A forced mechanical draft system involves a blower directing a flow of air at the fill media 6 while an induced mechanical draft system involves a pulling a flow of air up through or across the fill media 6.
As best illustrated in FIG. 2, another type of cooling tower does not include a fill media. Instead, water distribution equipment (such as a shower head) allows the cooling water to shower directly down into the basin 3.
There are at least three types of losses of cooling water from the cooling circuit. First, a portion of the trickling water, in the form of droplets or films of water, evaporates into the air. The latent heat of vaporization is removed from the non-evaporated portion of the cooling water thereby cooling it. Second, drift (sometimes called windage) is produced by a flow of air 4 carrying droplets of water out a top 2 of the cooling tower 1. The droplets impinge against a surface of a drift eliminator 10 so that some of the droplets that would otherwise be carried out the top 2 of the cooling tower will instead drip down over the fill media 6. Third, blowdown 12 is performed continuously or performed periodically when the cooling water in the basin 3 becomes too concentrated.
During this process a significant amount of water is lost through evaporation. As a result, dissolved minerals in the cooling water remaining after evaporation increase in concentration. As the concentration of a given mineral increases past its saturation point, scale (accretions of certain precipitated minerals) may start to form inside pipes, heat exchangers and various components of the cooling circuit. For example, an increase in calcium (Ca2+), will promote calcium carbonate (CaCO3) scale formation—the most common type of scale.
Over a period of time, scale build-up reduces the efficiency of heat transfer between a heat exchanger and the cooling water and restricts the flow of cooling water through the cooling circuit. As a result, scaling increases the operating costs, because more and more energy is needed to achieve a constant overall heat transfer rate in the cooling water system. If it is allowed to become uncontrolled, scaling can also result in a costly shutdown of the associated facility in order that excessive amounts of scale can be removed from equipment in contact with the cooling water.
To make up for losses from evaporation in the cooling tower, drift and blowdown (discharge of an amount of cooling water increasingly concentrated in various organic and inorganic constituents), makeup water is added to cooling circuits. Installations can use various types of water as their makeup water such as surface waters (lakes, rivers etc.), water from aquifers, process waters, industrial waters, or seawater. In some cases, the properties of the makeup water, such as temperature, pH, alkalinity, Ca2+ and magnesium (Mg2+) hardness, conductivity, total dissolved solids (TDS), etc., can vary significantly when compared on a weekly, daily, or even hourly basis. Changes in these properties can have an impact on scale formation. Operating parameters within a cooling circuit can also fluctuate either because of varying heat load, flow rates, meteorological conditions, etc. These changes in cooling circuit operating parameters can also have an impact on scale formation. While the scaling mechanism is well known in tightly controlled conditions and several schemes have been developed to control it, the above combined impacts increase the difficulty of controlling scale formation.
Several solutions exist to inhibit, prevent, and/or remove scale from cooling circuits and towers. The most common solutions are physical removal of the scale, addition of scale inhibiting chemicals, and addition of pH change agents to dissolve the scale. One type of pH agent used in cooling circuits include mineral acids such as hydrochloric acid (HCl) or sulfuric acid (H2SO4). However, mineral acids have a high degree of hazardousness and corrosivity thereby increasing the human and capital risk involved in handling it. In the case of H2SO4, it increases the sulfate SO42− concentration in the blowdown thereby potentially subjecting discharge of blowdown to environmental regulation. Softening of the makeup water and/or recirculation water is yet another option, but this quite often involves expensive equipment having intensive maintenance needs.
Carbon dioxide (CO2) is a less commonly used pH change agent. Although there are several technical and environmental advantages to using CO2 as a pH change agent (lower degree of hazardousness and corrosivity and reduced sulfate discharge). The typically larger amount of CO2 consumed vs. mineral acid consumed is especially noticeable when cooling circuit operates at a high concentration factor (the ratio of the concentration of a particular chemical constituent in the circulating water versus that in the makeup water) and/or when the cooling water has a high Ca2+ content and/or when the cooling water has a high alkalinity. Moreover, CO2 consumption is high due to losses in the cooling tower.
Thus, there is a need in the field of cooling water circuits for a better solution for preventing, inhibiting, or removing scale.